Semiconductor unit and method of making it



1958 G. L. SCHNABLE ET AL 2,857,560

SEMICONDUCTOR UNIT AND METHOD OF MAKING IT Filed Dec. 20, 1955 F11 1. F74. J. F74. 4.

V EN TORS BY Mf/EA 519001145X) United States Patent Gfltice.

2,857,560 "Patente'd Oct. 21, 1958 .ApplicationDecemberZtl,I955, SerialNo. 554,194 zfiiclaims. .(Cl...317235) .Thisflinvention :relates to-methoids aof and meansrfor :plasticzzencapsulation, or 'so-called' .fpotting, of semiconiduetorrlevices; suchias diodes or. transistors. Potted semirconductors "have :a numbersof important "advantages, in- .-.cluding.-stabilityiof operating characteristics during life zdf :the device, capability-of withstanding severe shock J without deleterious .effects, :and susceptibility of production, inquantity while preserving uniformity and predictability of operation. -Barticularly,;:tl1e potting technique .as has been demonstrated :by :much use :of the same by applicants assignee-minimizes the possibility of contaminationof jthe transistorrdevice :during use, 'andvmaintains required standards ":of electrical? insulation.

:In-accordance with .suchztechnique the components of a semiconductor device-arerigidly held. in .a predetermined critical .prelationship by -embedding said devicein a socalled ,plastic material-such, for :example, .as .a synthetic resin comprising-a high polymer product, this material being sealed-Within ametal .shield, or can.

For certain types of use to whichthe semiconductor. device may be, put it is important to insure maximum dissipation of heat generated in the. device. and. more particular- .ly.dissipation of :such heat .by transfer thereof through -the.potting.material. .Withthis requirement in mind, it is the primary object-:of thepresent invention to effect a substantial improvement inithe transfer of heat from: the semiconductor device through the potting material to the shield member-or can in which-the latter is encapsulated, and yet to utilize all of the advantageous electrical and mechanical characteristics afforded by potting.

Withmore particularity, our invention has as an objective the provision of methods of and means for potting semiconductor :devices in such manner as to combine the properties of high heat and lowelectrical conductivity. ln=accordance with a feature of the invention these object-ives are realizedby utilizing a new and improved potting mixture, and by providing a novel methodof mechanically treating said mixture and the semiconductor device embedded therein.

.Tothe foregoing ends,.and first briefly and generally described, our invention, in the method aspect, comprises: providing, in a suitable can,.a mixture of plastic material ina fluid state, with heat-conductive filler particles relatively thinly dispersed in said material; inserting .the semiconductor device, including its electrode means, into the mixture; then concentrating filler particles of the mixture in a relatively limited region in the can, which region is in contact with such electrode means as well as with the can; and finally converting the material into a substantially solid substance. Viewed in the apparatus aspect, our invention provides a transistor potting structurecomprising a relatively solid plastic body in a metal can, one part of the plastic body having a great number of fillerparticles embedded therein and forming an electrically insulating but thermally transmissive shell connecting the collector electrodes with the can. Another part of the pastic body, 'free of filler particles, may, at least in certain embodiments of theinvention, surround and insulate lead-wires of the device; and the metal can 'may be electrically insulated fromthe entire semiconductor device, while being thermally connected to heatgenerating electrode portions thereof.

Details of the'fabrication process and of the unit formed thereby will be understood more clearly upon astudy of the description of preferred embodiments, which follows, with reference to the :drawingappended hereto, wherein- Fig. 1 shows onephase or the fabricating process;

Fig. 2 shows a subsequentphase of this process, on an enlarged scale;

Fig. 3 showsa third phase of this process; and

Fig. 4 shows a fourth-phase of the fabricating process and a cross section'throughxthe completed unit.

.Referring first to Fig. 1,1the .fabrication starts by providing a semiconductor device, shown as a transistor 10 with collectorwire'11,..emitter 'wireil2 and'base wire 13, portions ofsuch-wires ".beingisealed in a glass bead 14 surrounded-by a metalsleeve 15. There is also provided a metal 0211116 containing :a. body :17 of a certain type of plastic material in fluid condition, that is,.in form of a liquid or @paste. .For: example, :a :loW-molecular-weight polyethylene compound maybe often used as such a materialyin this casethe subsequentsolidification or curing is a thermoplastic operation. Otherexamples'of thermoplastic materials which=mayssometimes be used advantageously forathe' present .1purposes are: paraffin, polychlorotrifluoroethylene, .and polystyrene resin. Examples of thermosetting materials-which may beused are: silicone rubber, epoxy I'ESziHS,.'ZlHd vinyl functional dimethyl silicones. :These and some other-suitable-plastic materials 'after the curing and-havingrelativelylittleetfect andno impairing efiectaupon' theoperating characteristics of the semiconductor during their-contact with. they semiconductor at the curingtemperatures-of the-plastic and at the operating temperatures of the semi-conductor. Another such material,-. of .the thermosettingatype, isused by. our assignee under .thetrade narne Lanco No. 6.

A greatnumber of small fillergparticles are uniformly .and'relatively dilutely dispersed in this plastic, as shown by uniformly distributed stipplinginthe figure. under dis- .cussion. We haveobtained asrneasure of .success'with a variety of. filler materials, including among others, ,aluminum .oxide, ;surface-.oxidized .aIuminum, titanium dioxide, silicon rlioxide, red lead oxide and zirconium. orthosilicate. Best, resultshave been. obtained with powdered and surface-oxidized silicon .carbide. Advantageously this materialisintroduced into the .fluid plastic in very finelypowderedparticle state .so.that..the mass of filler particles may. intimately contactthesurfaces of the semiconductor-device. The material .then forms a SIOWrSEttiiIIg .or nomsettling-suspension .in .thefluid plastic. It must further beprovided in such a substantial butlimited amount z-asto :appreciably raise the thermal conductivity of the mixture-at leastaafterrconcentrationof the fillers, .but still to leaveit possible, before such-concentration to lower. the semi-conductondevice into the mixture under suitable temperatureeconditions, without injury to parts such as the delicate semiconductor hodywandits cat whiskers 18, ;1=9:(-Fig. 2).. 'The'fluid mixture provided, prior :to concentration, will'then retain a certain degree of fluidity. It seems to 'be most advantageous, for the different operations mentione'd, if-said fillers and plastic are mixed in an approximate ratioranging between 60 to 40 and 70 to 30, byweighty'the exact selection dependingon features 'su'ch as the sizes and shape of the filler particles used. "Wehave obtained'particularly good .tion.

3 Lanco No. 6 with surface-oxidized silicon carbide particles having screen sizes between 300 and 500.

Fig. 2 shows the condition of the parts and particles on completion of the transistor inserting and submerging operation; a substantially uniform mixture of fluid plastic vand filler particles surrounds the transistor and practically fills the can to the top.

In order to allow good transistor operation, and particularly good heat dissipation, some or all of the filler particles of the original mixture must first be concentrated, in a limited portion 21 of the fluid plastic body 17, that is, removed from another portion 22 of said body. The portion or zone 21 of particle concentration, as shown, contacts and surrounds certain electrodes of the transistor, particularly the collector electrode 20 thereof and desirably also surrounding portions of the semiconductor itself; typical semiconductor materials being better conductors of heat than typical electrode materials are. On the other hand, said zone of particle concentration is contacted and surrounded by an inside surface of the metal can. Thus we establish the condition shown in Fig. 3, wherein, as shown, most or substantially all of the particles are disposed in the lower part 21 of the plastic body 17 and few or substantially none remain in the upper part 22 of said body.

By settling the particles, such concentration has been applied to the filler particles mentioned, in said fluid plastic, in about one week. Substantially the same degree of concentration and the same beneficial results have been achieved by centrifuging the mixture for live minutes, under an acceleration of 20,000 g., the centrifugal force being applied in the direction of the arrow in Fig. 3. centrifuging is also desirable in many cases, and sometimes even required, in order to test the semiconductor device as to mechanical strength against vibration and shock load; thus the present centrifuging operation causes no added increase in the cost of the equipment required for this process. The concentrating must be performed at a relatively raised temperature if one of the thermoplastic materials is used.

The concentrated arrangement of the filler particles in the plastic is then made permanent, incident to the known operation of converting the plastic into a solid, at temperatures suitable for this purpose. The exact thermoplastic or thermosetting procedures, temperatures, periods, etc. need not be specified herein. In many applications of the present method, the heating up of the entire mixture must also be controlled to avoid overheating of local portions thereof, which would injure the semiconductor or the potting compound, or both. Sometimes the heating must be performed with particular care, in order to avoid redispersion of the concentrated filler particles at the start of the curing operation.

Fig. 4 shows the condition of the involved parts and particles after curing. For present purposes this condition may be considered as the final stage of the process, except that it is also necessary to seal the aperture 23 between can 16 and stem 15, which can be done before or after curing, depending on features of no importance for this invention. Thus Fig. 4 also shows the substantially completed unit. Substantially all of the filler particles are embedded in solid plastic in what is shown as the lower part, 21, of the can 16. They form a concentrated mass of plastic-embedded particles in this part, extending directly from the middle and lower portions of the transistor to the can.

This structure allows rapid transmission of large amounts of heat from heat-generating parts of the transistorparticularly from the collector electrode 20 in the middle of the transistor-to the metal of the can 16, and thereby, as suggested by arrows, to ultimate dissipa- A typical embodiment of the new unit, connected to an infinite heat sink to keep the can at room temperature. showed collector temperature rises of only about .005 degree centigrade per milliwatt of collector 4 energy dissipation; Whereas an identically constructed and connected transistor unit, potted in an identical plastic but without fillers, showed corresponding temperature rises of about .043 degree; said dissipations ranging up to 700 milliwatt and the starting temperature being 27 degrees centigrade.

The present potting unit also showed advantageously high electrical resistivity and low permeability for contaminants, as well as adequate mechanical, electronic and other characteristics, required for proper semiconductor operation.

While we have described only one basic manner of performing the process and only one basic product thereof, it should be understood that the details thereof are not to be construed as lirnitative of the invention, except insofar as set forth in the following claims.

We claim:

1. A method of plastic encapsulation of a semiconductor device, comprising the steps of submerging the device, including electrode means thereof, in a can filled with a relatively dilute mixture of fluidv plastic material and heat-conductive filler particles; then concentrating substantially all of said particles of the mixture in a relatively limited region in the can, which region is in contact with such electrode means and with the can; and then converting the mixture into a substantially solid substance of low electrical conductivity, thereby permanently locking the concentrated particles in said region.

2. A method as described in claim 1 wherein the concentrating is performed by centrifuging.

3. A method as described in claim 1 wherein the concentrating is performed by settling.

4. In a method of potting a semiconductor device in a can filled with liquid plastic of low electrical conductivity, the steps of first dispersing thermally conductive filler particles throughout the liquid in the can, so as to form a mixture dilute enough to allow insertion of the device into the mixture without damage to the device; then concentrating substantially all of said particles in, and-distributing them throughout, a portion of the mixture extending directly from electrode means of the device to the can; and then solidifying the plastic.

5. A method as described in claim 4 wherein the ratio of the dispersed particles to the liquid plastic is in the approximate range between 60:40 and 70:30 by weight.

6. A method as described in claim 4 wherein the limited portion of the mixture, containing the concentrated filler particles, amounts to about one-half of the total volume of the mixture in the can.

7. A method as described in claim 4 wherein the filler particles substantially consist of silicon carbide.

8. A method as described in claim 7 wherein the filler particles have screen sizes between about 300 and 500.

9. A method as described in claim 4 wherein the plastic has high dimensional stability during the solidifying operation.

10. A method as described in claim 4 wherein the plastic is of a type having no impairing efiect upon the operating characteristics of the semiconductor device during contact with the semiconductor at the curing temperatures of the plastic and at the operating temperatures of the semiconductor.

11. A method a described in claim 4 wherein the plastic is heated during said concentrating operation.

12. A potted semiconductor unit, comprising a body of solid plastic material of low electrical conductivity; a semiconductor device including electrode means connected thereto, embedded in said body; a metal can surrounding said body; and heat-conductive filler particles embedded in said body, at close distances from one another, throughout a zone which constitutes only a limited portion of said body and extends from an outside surface of at least one of the electrode means directly to an inside surface of the can.

13. A unit as described in claim 12 wherein said particles substantially consist of silicon carbide.

14. A unit as described in claim 12 wherein said particles have screen sizes of about 300 to 500.

15. A unit as described in claim 12 wherein the semiconductor device is secured to a stem, the unit com prising in longitudinal alignment: the stem; an integral portion of the body of plastic, relatively free of said particles; and said limited portion of said body, with said particles embedded therein.

16. A unit as described in claim 15 wherein the semiconductor device has lead wire means extending through the integral portion of the body of plastic.

17. A unit as described in claim 12 wherein the semiconductor device is a transistor having a collector electrode and said limited portion of said plastic body containing said filler particles extends from said collector electrode to the can.

18. A unit as described in claim 12 wherein the semiconductor has electrodes of the surface barrier type, in said limited portion of said plastic body.

19. A unit as described in claim 12 wherein the metal can is electrically insulated from the semiconductor device therein.

Reterences Cited in the file of this patent UNITED STATES PATENTS 1,959,934 Smith May 22, 1934 1,994,534 Robinson Mar. 19, 1935 2,034,434 Heintz Mar. 17, 1936 2,375,058 Wiegand May 1, 1945 2,636,073 Clarke Apr. 21, 1953 2,666,073 Slade Jan. 19, 1954 2,758,261. Armstrong Aug. 7, 1956 FOREIGN PATENTS 671,473 Great Britain Apr. 1, 1949 742,413 Great Britain Dec. 30, 1955 

